.TH runqslower 8  "2016-02-07" "USER COMMANDS"
.SH NAME
runqslower \- Trace long process scheduling delays.
.SH SYNOPSIS
.B runqslower [\-p PID] [\-t TID] [min_us]
.SH DESCRIPTION
This measures the time a task spends waiting on a run queue (or equivalent
scheduler data structure) for a turn on-CPU, and shows occurrences of time
exceeding passed threshold. This time should be small, but a task may need
to wait its turn due to CPU load. The higher the CPU load, the longer a task
will generally need to wait its turn.

This tool measures two types of run queue latency:

1. The time from a task being enqueued on a run queue to its context switch
and execution. This traces ttwu_do_wakeup(), wake_up_new_task() ->
finish_task_switch() with either raw tracepoints (if supported) or kprobes
and instruments the run queue latency after a voluntary context switch.

2. The time from when a task was involuntary context switched and still
in the runnable state, to when it next executed. This is instrumented
from finish_task_switch() alone.

The overhead of this tool may become significant for some workloads:
see the OVERHEAD section.

This works by tracing various kernel scheduler functions using dynamic tracing,
and will need updating to match any changes to these functions.

Since this uses BPF, only the root user can use this tool.
.SH REQUIREMENTS
CONFIG_BPF and bcc.
.SH OPTIONS
.TP
\-h
Print usage message.
.TP
\-p PID
Only show this PID (filtered in kernel for efficiency).
.TP
\-t TID
Only show this TID (filtered in kernel for efficiency).
.TP
min_us
Minimum scheduling delay in microseconds to output.
.SH EXAMPLES
.TP
Show scheduling delays longer than 10ms:
#
.B runqslower
.TP
Show scheduling delays longer than 1ms for process with PID 123:
#
.B runqslower -p 123 1000
.SH FIELDS
.TP
TIME
Time of when scheduling event occurred.
.TP
COMM
Process name.
.TP
PID
Process ID.
.TP
LAT(us)
Scheduling latency from time when task was ready to run to the time it was
assigned to a CPU to run.
.SH OVERHEAD
This traces scheduler functions, which can become very frequent. While eBPF
has very low overhead, and this tool uses in-kernel maps for efficiency, the
frequency of scheduler events for some workloads may be high enough that the
overhead of this tool becomes significant. Measure in a lab environment
to quantify the overhead before use.
.SH SOURCE
This is from bcc.
.IP
https://github.com/iovisor/bcc
.PP
Also look in the bcc distribution for a companion _examples.txt file containing
example usage, output, and commentary for this tool.
.SH OS
Linux
.SH STABILITY
Unstable - in development.
.SH AUTHOR
Ivan Babrou, original BCC Python version
Andrii Nakryiko, CO-RE version
.SH SEE ALSO
runqlen(8), runqlat(8), pidstat(1)
